Aluminum metal is produced in reduction cells by the Hall-Herolt process by reducing alumina electrolytically with cryolite. The cells in which this process is performed are lined with carbon, with the carbon lining forming the cathode for the process.
During the campaign of a reduction cell, this potliner carbon becomes contaminated with numerous materials, including cryolite, alumina, aluminum, sodium, sodium oxide, sodium carbonate, sodium hydroxide, sodium fluoride, aluminum nitride and aluminum carbide. When the cell is rebuilt, this potliner carbon is removed, with the removed material being identified as spent potliner.
At one time, it was considered common practice to discard this spent potliner as waste. However, in order to increase the economics of the reduction process, it has become increasingly important to recover and recycle this material.
It is known that spent potliner can be crushed and mixed with carbonaceous aggregate and pitch to form a paste, with this paste being employed as the anode of a Soderberg reduction cell. In a Soderberg cell, the anode is constantly being consumed at its bottom within the molten alumina-cryolite bath and at the same time is being replenished by additions of anode paste at its top. Over time, the anode paste is heated and baked such that by the time it reaches its working position at the bottom of the cell, it has become a solid monolith.
Unfortunately, it has been found that if an amount greater than about 5 to 10% of the total aggregate is replaced by untreated spent potliner, the resulting baked carbon anode tends to have a structure containing numerous cracks and does not have sufficient mechanical strength for use as a Soderberg anode.
It has also been attempted to replace up to 25% of the total aggregate with spent potliner in forming a Soderberg anode. However, in this case, it was found that the anode was consumed during electrolysis at a greater than normal rate.
Thus, previous attempts to utilize spent potliner as a feed stock for Soderberg anodes have met with little success. It remains desirable, however, to replace as much of the carbonaceous aggregate in a Soderberg anode paste with spent potliner as possible, for obvious economic reasons.